ACTIVE NOISE-CANCELLATION SYSTEM FOR AUTOMOTIVE MUFFLERS

Abstract: In an active control noise-cancelling muffler system, a substantially complete acoustic and mechanical decoupling of the noise-cancelling signal pipe from the gas exhaust pipe (13) is achieved. The noise-cancelling signal delivery pipe is physically isolated and separate from the gas exhaust pipe, and is mounted separately. The outlet ends of both pipes are essentially coplanar. Using pressure sensors (17, 18) on the tubes, the system also accurately and continuously electronically replicates the mixing of the exhaust noise and the noise-cancelling acoustic energy that goes on in the space immediately beyond the two outlets. This electronic signal is a useful alternative for a direct measure of the resultant two acoustic waves when they mix in the space beyond the tube outlets, and allows the system to continuously estimate the degree of success of noise cancellation without having to measure it directly when impractical. The system (30) uses measures of pressure (17, 18) and temperature (36, 37) of the two tubes to continuously adjust a transducer drive signal that drives the sum of the pressures at the two tube outlets toward zero. An advantageous algorithm for the control process is identified.

Description: ACTIVE NOISE-CANCELLATION SYSTEM FOR AUTOMOTIVE MUFFLERS FIELD OF THE INVENTION This invention relates generally to active cancellation of unwanted acoustic energy in a physical environment; and, more specifically, to improvements in active noise-cancelling automotive mufflers and muffler systems. BACKGROUND OF THE INVENTION Active noise-cancelling muffler systems used with internal combustion engines typically include means for monitoring selected parameters of the exhaust system and gas flow; and using the parameters, developing a noise-cancelling acoustic waveform. The "counter-acoustic wave" typically is formed first as an electrical waveform generated by a controller. The controller may be a computer or chip driver connected to an amplifier for a transducer that generates the cancelling signal. The cancelling wave and the exhaust gas energy continuously subtractively combine to effect the desired noise reduction. In active noise-cancelling muffler systems, it is necessary to control the noise-cancelling acoustic signal both spatially and temporally so that the negative- going pulses of the cancelling wave coincide with the positive-going pulses of the exhaust gasses. The prior art teaches a variety of control strategies, using various physical structures to contain the transducer and launch the counter-acoustic wave. However, the physical design of the systems as well as the efficiency of the control signal leave much to be desired in terms of cost and reliability. The controller requires accurate information as to the upstream exhaust gas reference pressure in order to generate a useful transducer input signal. One problem with many such systems of the prior art is that acoustic or mechanical coupling occurs between the counter-acoustic wave generator and the exhaust system of the IC engine. Readings of the exhaust gas reference pressure that are perturbed by mechanical or acoustical coupling from the noise cancelling apparatus, complicate the controller's function by requiring more complex and time-consuming computations to compensate for the perturbations. If the actual on-going reference gas pressure fluctuations is substantially obscured by such perturbations, the functionality of the system may be defeated altogether. A continuously effective noise reduction system requires constant adjusting of the cancelling waveform to changing conditions which include exhaust temperature, frequency and amplitude. Ideally, despite the changing conditions, the exhaust gas acoustic energy is driven by the cancelling waveform toward zero at all times. The degree of success in achieving full cancellation depends in part on continuously measuring the actual noise reduction occurring at the exhaust pipe outlet. The measurement of noise reduction is critical in determining controller inputs that will cause the transducer to continuously drive the exhaust gas noise to zero. In most prior art active control noise-cancelling vehicular muffler systems, because the c-ancelling waveform and the exhaust noise waveform are acoustically and mechanically coupled in the same pipe, it is straightforward to measure the actual noise reduction simply by placing a single sensor at the common outlet. If, on the other hand, the noise-cancelling generator and the exhaust pipe are physically decoupled, the prior art expedient for measuring actual noise reduction is not available. Further, it is not practical for cost and reliability reasons to place a pickup microphone in the space beyond the exhaust pipe outlet to measure the actual reduced-noise exhaust gas amplitude. A practical measure of the spatial and temporal components of the noise-reduced exhaust gas pressure for use with decoupled systems is needed. SUMMARY OF THE INVENTION Substantially complete acoustic and mechanical decoupling of the noise- cancelling signal generating pipe from the gas exhaust pipe is achieved in accordance with the invention by providing a noise-cancelling signal delivery pipe which is entirely physically isolated and separate from the gas exhaust pipe. In one embodiment, the outlet end of the noise-cancelling pipe is placed side-by-side with the outlet end of the exhaust pipe. The noise-cancelling apparatus is a pipe closed at its far end where the transducer is mounted. The outlet end of this pipe advantageously is essentially coplanar with the outlet of the muffler exhaust pipe. The two pipes are closely spaced, but not directly mechanically coupled. The noise- cancelling pipe is formed to be as short as possible. A method and apparatus is used for accurately and continuously electronically mimicking the mixing of the exhaust noise and the noise-cancelling acoustic energy that goes on in the space immediately beyond the two outlets where the far-field acoustic cancellation takes place. Using this approach, a highly efficient noise-cancellation signal may be generated using any of several available algorithms. Pressure sensors are respectively placed in the interior of the exhaust pipe and the noise cancellation tube, just inside the outlet mouths. By putting the two end sensors into the interior of the respective sidc-by-side tubes, there is high assurance that the respective pressure readings measure only the gas exhaust pressure and the noise cancellation signal pressure respectively. Neither reading is influenced by cross- coupling of the acoustic energy from the other. Further, ambient noise in the immediate environment has little, if any, effect on the readings. DESCRIPTION OF THE DRAWING FIG. 1 is a schematic diagram of a general automotive exhaust apparatus; FIG. 2 is a schematic diagram of an automotive exhaust apparatus in accordance with the invention; FIG. 3 is a schematic block diagram of the apparatus and noise- cancelling control system of the invention; FIG. 4 is a diagram illustrating the mixing in space of a noisy and noise-cancelling waveform; FIG. 5 is a flow chart illustrating the noise-cancelling process of the invention; and FIG. 6 is a second physical arrangement of plural separate noise- cancelling tubes in relation to the exhaust pipe. DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENT As seen in FIG. 1, the exhaust system 10 of a typical automotive vehicle consists in part of a muffler 12 and a exhaust pipe 13 with an outlet 14. The system is mounted on the vehicle chassis, denoted 11 usually with noise isolation mounts 15. As seen in FIG. 2, a noise-cancelling system 20 consists of a closed end chamber 21 connected to a noise-cancelling tube 22, gas pressure sensors 16, 17, 18, a signal controller 30 and a transducer and amplifier 24, 40. The tube 22 of the inventive system has an outlet 23. The two outlet ends 14, 23 are disposed closely adjacent to each other, advantageously in coplanar relation. Transducer 24 is mounted in the interior of the closed end of tube 22. Advantageously, a transducer with relatively little nonlinearity characteristic should be used in order to avoid introducing any distortion or harmonic content in the output which must be compensated for in generating the control signal. Additionally, the acoustic design of the combined closed-end chamber 21 and noise-cancelling tube 22 should be constructed to minimize the transmission of non- linear by-products of the transducer to the atmosphere through the outlet 23. Tube 22 and chamber 21 are connected to the vehicle chassis by isolation mounts 15 as in FIG. 1. These mounts are the only mechanical or structural connection the noise-cancelling system 20 has with the exhaust system 10. That, and other expedients to be described, make the gas exhaust and the noise-cancelling systems substantially decoupled acoustically. It is essential that the tube 22 be as free as possible of any acoustic energy other than the precise counter-acoustic waveform generated by transducer 24. Any resonances characteristic of the tube 22 may amplify small but significant harmonic acoustic energy produced by the transducer 24, and therefore are detrimental. To eliminate as much as possible the natural resonant frequencies in the tube 22, a suitably shaped acoustic cavity 25 connected to the tube 22 may be provided. However, by constructing the tube 22 to be less in length than about 0.25 meters, or more generally less than about one-half of the wavelength of the highest frequency to be cancelled, the resonances of tube 22 will occur at frequencies higher than those which are to be cancelled from the exhaust gas stream resonances. Referring now to FIG. 3, a first gas pressure sensor 16 is mounted upstream in the exhaust system 10 at a point forward of the exhaust outlet 14. That point should be located at a distance from the outlet which is greater in length than a half wavelength of the highest frequency to be eliminated. Sensor 16 provides an early and on-going measure of the exhaust gas wave in transit. A second exhaust gas pressure sensor 17 is mounted just inwardly of the outlet 14 of exhaust pipe 13. A third pressure sensor 18 is mounted just within the outlet 23 of delivery tube 22. The outputs of sensors 17 and 18, in accordance with one aspect of the invention, are electronically summed in summer 19. It may be useful to filter and weight the reading of sensor 17 in order to compensate for sound radiation differences due to temperature, gas flow, and diameters of the exhaust and noise-cancelling tubes, thereby improving the noise cancellation in the far field. The weighting may take place in an adjustment under the control of an operator; or the weighting may occur in the controller 30. - - Controller 30 may comprise a computer or custom chip. It receives the output of summer 19 or alternatively the independent outputs of sensors 17 and 18 output of sensor 16. Controller 30 includes a digital signal processor or the equivalent to calculate the control signals to the amplifier 40 for the transducer based on the several inputs of gas pressure. In fashioning the electrical input to amplifier 40, various inputs besides gas pressure may be desirable to take into account in controller 30, such as engine RPMs. In particular, temperature information can be critical for proper shaping of the counter-acoustic waveform. When the vehicle engine is first started, the exhaust gases are relatively cool; but, as the engine warms up or takes on load, the exhaust gases become intensely hot, reaching temperatures of several hundred to 1000 degrees F. at the pressure sensor 17. Of course, with elevated heat, the afore¬ mentioned filtering and weighting of the reading of sensor 17 is affected. The noise-cancelling performance of the system 20 is improved by adding a temperature sensor 36 adjacent to pressure sensor 17 and a temperature sensor 37 adjacent to sensor 18. The temperature readings in conjunction with either measures of IC engine RPM or exhaust gas velocity, may be used to compute the weighting factor for the reading of sensor 17 to account for the sound radiation differences between the cancellation tube and the exhaust pipe. For identical tube diameters, this factor is a magnitude scaling term across the frequency band of interest; and may be experimentally determined for each engine exhaust and cancellation tube geometry. This factor is then included in the control algorithm as a table look-up value or as an empirical equation representing effects across the frequency range of interest Means in controller 30 are provided to vary the controller output as a function of the incoming readings of the value measured by summer 19. This sum is maintained continuously at a minimum possible amount. As a result, as illustrated in FIG. 4, the acoustic mixing of the exhaust gas wave 32 and the counter-acoustic waveform 33 in the space denoted 31 immediately beyond the two outlets are substantially cancelling. One strategy for continuously minimizing the value in summer 19 is to perform an algorithm in controller 30 known as the "filtered X least means square algorithm". This algorithm is fully described in Adaptive Signal Processing by B. Widrow and S. D. Stearns, Prentice Hall, Englewood Cliffs, New Jersey, pp 288-297 1985 and is hereby incorporated by reference. The "filtered X" algorithm may be practiced in software written into, for example, a conventional digital signal processor 34 in controller 30. In practicing the filtered X-algorithm, the time advanced reference pressure detected by sensor 16 is sampled approximately every 250 microseconds. This signal is then filtered through a recursive, discrete-time filter representation of the acoustic cancellation path to form a filtered version of the pressure reference signal. The filtered reference is next correlated with the summed error signal for the current sample period, and scaled by a convergence gain factor μ. This scaled, correlated signal becomes the adaptive weight update in the "filtered X" algorithm. From the weight update computation, the drive signal for transducer 24 is determined by adding the current weight update to the weight update computed for the previous sample period. This sum becomes the transducer drive signal that is fed to amplifier 40. The weight updating and resulting varying of the transducer driver signal is continuous. Referring again to FIGS. 2 and 3, because of the substantial decoupling of the exhaust pipe 13 and the delivery tube 22, the readings picked up by pressure sensor 16 are uninfluenced by the output of transducer 24. As a result, it is not necessary in practicing the algorithm in controller 30 to take into account any modulations of the gas exhaust energy caused by the noise-cancelling wave. Further, the isolation of the two tubes allows controller 30 to track slow changes in the transfer function of the noise-cancelling tube much more simply than if the two tubes were directly mechanically coupled. By accessing either the output of the noise-cancelling tube sensor 18 before the signal enters the summer 19 or the output of the summer 19 and the output of controller 30, a reliable and continuous on-line estimate of the noise-cancelling tube transfer function can be made without interference from the exhaust noise. This is an advantage over some prior art automotive exhaust noise-cancelling systems which use a pilot signal to identify this transfer function in order to enable the noise-cancelling signal to be responsive to changes in the characteristics of the delivery tube. A pilot signal necessarily adds further noise to the system output, however, which is counter to the purpose of noise-cancelling muffler systems. It also may be desirable to monitor the temperature in the noise- cancelling tube to aid in the on-line estimation of the transfer function characteristics of the noise-cancelling tube. The monitored temperature information of the noise- cancelling tube may be used to select, from a library of pre-determined transfer functions contained in a database in controller 30, an initial estimate of the transfer function appropriate for the current measured temperature. This expedient is an effective way to initiate running the algorithm; and has the advantage of providing more rapid estimation of the required transfer function than could otherwise be achieved without the temperature measurement. A further advantage of the acoustic isolation and separation of the exhaust pipe 13 and the tube 22, is that temperature excursions occurring in the exhaust pipe at least do not require instantaneous compensating adjustment of the transducer 24 amplitude or phase due to effects of exhaust gasses on the acoustic cancellation path transfer function, as would be the case if the delivery tube were directly mechanically coupled into the exhaust pipe. The principles of the invention have been illustrated by the example of a single noise-cancelling tube mounted to the side of the gas exhaust. However, the principles are applicable to substantially any configuration of noise-cancelling tubes. One such variation is shown in FIG. 6, as a series of noise-cancelling tubes 22 symmetrically arrayed around the gas exhaust pipe 13. Further configurations can readily be envisioned by persons skilled in the art

Claims: Claims: 1. In an internal combustion engine exhaust gas system comprising an exhaust pipe 13 having an outlet 14, a noise-cancelling signal delivery tube 22, said delivery tube 22 bemg .separate from but located adjacent to said exhaust pipe 13 and having its outlet 23 disposed adjacent to said exhaust pipe outlet 14 and a transducer 24 mounted in said deliveiy tube far end for generating an exhaust gas noise- cancelling signal; apparatus for reducing the acoustic energy in the exhaust stream at said outlet, characterized in that: a first gas pressure sensor 16 is mounted upstream in said exhaust pipe for generating a reference gas pressure; second and third gas pressure sensors 17, 18 are respectively mounted at the outlet ends of said exhaust pipe and said delivery tube; means 30 including readings from said second and third gas pressure sensors 17, 18 are provided for generating an electronic signal replicating the combined waveform of the exhaust noise and noise-cancelling waveforms in the space 31 immediately beyond said tube outlets 14, 23; and means 34, 40 responsive to readings from said first sensor and to said indicia of said replicating electronic signal are provided for generating a drive signal for said transducer 24. 2. Apparatus in accordance with claim 1 , wherein said electronic replica signal generating means 19 further comprises means for summing the readings of said second and said third pressure sensors 17, 18, and said apparatus further comprising: means for periodically adjusting said transducer drive signal to a level that maintains said sum in means 19 at a minimum value. 3. Apparatus in accordance with claim 2, wherein said second and third pressure sensors are respectively placed in the interior of said exhaust pipe 13 and said noise-cancelling delivery tube 22 at said tube outlets. 4. Apparatus in accordance with claim 3, wherein said tube outlet ends 14, 23 are disposed closely adjacent to each other and are in substantial co-planar relation. 5. Apparatus in accordance with claim 4, wherein said noise-cancellation delivery tube 22 is formed to a length measured from its said far end to its said outlet equal to substantially one-half of the wavelength of the highest frequency present to be cancelled in said exhaust stream. 6. Apparatus in accordance with claim 5, wherein said means for generating said transducer drive signal further comprises: means 30 for periodically sampling said reference pressure sensor 16 to generate a pressure reference signal; means 34 for filtering said sample through a recursive, discrete-time filter representation of the acoustic cancellation path, thereby to form a filtered version of said pressure reference signal; means for correlating said pressure reference signal with the sum of said second and third pressure sensor readings for the current sample period, and for scaling the result by a convergence gain factor to create a current weight update number; and means 30 for currently adjusting the drive signal for said transducer 24 by adding the current weight update number to the weight update number computed for the previous sample period. 7. Apparatus in accordance with claim 6, further comprising: a temperature sensor 36, 37 disposed adjacent to each of said second pressure sensor 17 and said third pressure sensor 18; and means 30 responsive to the readings from said temperature sensors 36, 37 for adjusting the drive signal of said transducer 24 to continue to drive said sum of said first and said second pressures to a minimum value. 8. Apparatus in accordance with claim 7, further comprising: a plurality of noise-cancelling signal delivery tubes 22 symmetrically arrayed around said gas exhaust pipe 13.

Inventors: BROWNING DOUGLAS ROY, ZUNIGA MICHAEL ANTHONY, BROWNING, DOUGLAS, ROY, ZUNIGA, MICHAEL, ANTHONY
Applicants: AT & T CORP, BROWNING DOUGLAS ROY, ZUNIGA MICHAEL ANTHONY, AT&T CORP., BROWNING, DOUGLAS, ROY, ZUNIGA, MICHAEL, ANTHONY
Application year: 1994
Main classification: A61F11
IPC codes: B60K13, F01N1, G10K11